Space elevator
A space elevator for Earth would consist of a cable fixed to the Earth's equator, reaching into space. By attaching a counterweight at the end (or by further extending the cable upward for the same purpose), the center of mass is kept well above the level of geostationary orbit. Upward centrifugal force from the Earth's rotation ensures that the cable remains stretched taut, fully countering the downward gravitational pull. Once above the geostationary level, climbers would have weight in the upward direction as the centrifugal force overpowers gravity. (The height relative to the diameter of the Earth on the diagram is to scale. The height of the counterweight varies by design and a typical, workable height is shown. A space elevator is a proposed type of space transportation system. Its main component is a ribbon-like cable (also called a tether) anchored to the surface and extending into space. It is designed to permit vehicle transport along the cable from a planetary surface, such as the Earth's, directly into space or orbit, without the use of large rockets. An Earth-based space elevator would consist of a cable with one end attached to the surface near the equator and the other end in space beyond geostationary orbit (35,800 km altitude). The competing forces of gravity, which is stronger at the lower end, and the outward/upward centrifugal force, which is stronger at the upper end, would result in the cable being held up, under tension, and stationary over a single position on Earth. Once deployed, the tether would be ascended repeatedly by mechanical means to orbit, and descended to return to the surface from orbit.Edwards, Bradley Carl. The NIAC Space Elevator Program. NASA Institute for Advanced Concepts The concept for a space elevator was first published in 1895 by Wikipedia:Konstantin Tsiolkovsky. His proposal was for a free-standing tower reaching from the surface of Earth to the height of geostationary orbit. Like all buildings, Tsiolkovsky's structure would be under compression, supporting its weight from below. Since 1959, most ideas for space elevators have focused on purely tensile structures, with the weight of the system held up from above. In the tensile concepts, a space tether reaches from a large mass (the counterweight) beyond geostationary orbit to the ground. This structure is held in tension between Earth and the counterweight like an upside-down plumb bob. Space elevators have also sometimes been referred to as beanstalks, space bridges, space lifts, space ladders, skyhooks, orbital towers, or orbital elevators. On Earth, with its relatively strong gravity, current technology is not capable of manufacturing tether materials that are sufficiently strong and light to build a space elevator. However, recent concepts for a space elevator are notable for their plans to use carbon nanotube or boron nitride nanotube based materials as the tensile element in the tether design. The measured strength of these molecules is high compared to their densities and they hold promise as materials to make an Earth-based space elevator possible. The concept is also applicable to other planets and celestial bodies. For locations in the solar system with weaker gravity than Earth's (such as the Moon or Mars), the strength-to-density requirements are not as great for tether materials. Currently available materials (such as Kevlar) are strong and light enough that they could be used as the tether material for elevators there.Moravec, Hans (1978). [http://www.frc.ri.cmu.edu/~hpm/project.archive/1976.skyhook/papers/scasci.txt Non-Synchronous Orbital Skyhooks for the Moon and Mars with Conventional Materials]. Carnegie Mellon University. frc.ri.cmu.edu History ]] Early concepts The key concept of the space elevator appeared in 1895 when Russian scientist Konstantin Tsiolkovsky was inspired by the Eiffel Tower in Paris. He considered a similar tower that reached all the way into space and was built from the ground up to the altitude of 35,790 kilometers, the height of Wikipedia:geostationary orbit. He noted that the top of such a tower would be orbiting Earth in a geostationary orbit. Objects would attain orbital velocity as they rode up the tower, and an object released at the tower's top would also have the velocity necessary to remain in geostationary orbit. Tsiolkovsky's conceptual tower was a compression structure, while modern concepts call for a tensile structure (or "tether"). 20th century Building a compression structure from the ground up proved an unrealistic task as there was no material in existence with enough compressive strength to support its own weight under such conditions. In 1959 another Russian scientist, Wikipedia:Yuri N. Artsutanov, suggested a more feasible proposal. Artsutanov suggested using a geostationary Wikipedia:satellite as the base from which to deploy the structure downward. By using a Wikipedia:counterweight, a cable would be lowered from geostationary orbit to the surface of Earth, while the counterweight was extended from the satellite away from Earth, keeping the cable constantly over the same spot on the surface of the Earth. Artsutanov's idea was introduced to the Russian-speaking public in an interview published in the Sunday supplement of Wikipedia:Komsomolskaya Pravda in 1960, but was not available in English until much later. He also proposed tapering the cable thickness so that the stress in the cable was constant. This gives a thinner cable at ground level that becomes thicker up towards GSO. Both the tower and cable ideas were proposed in the quasi-humorous ''Ariadne'' column in Wikipedia:New Scientist, December 24, 1964. In 1966, Isaacs, Vine, Bradner and Bachus, four American engineers, reinvented the concept, naming it a "Sky-Hook," and published their analysis in the journal ''Science''. They decided to determine what type of material would be required to build a space elevator, assuming it would be a straight cable with no variations in its cross section, and found that the strength required would be twice that of any then-existing material including Wikipedia:graphite, Wikipedia:quartz, and Wikipedia:diamond. In 1975 an American scientist, Wikipedia:Jerome Pearson, reinvented the concept yet again, publishing his analysis in the journal Wikipedia:Acta Astronautica. He designed a tapered cross section that would be better suited to building the elevator. The completed cable would be thickest at the geostationary orbit, where the tension was greatest, and would be narrowest at the tips to reduce the amount of weight per unit area of cross section that any point on the cable would have to bear. He suggested using a counterweight that would be slowly extended out to , almost half the distance to the Wikipedia:Moon) as the lower section of the elevator was built. Without a large counterweight, the upper portion of the cable would have to be longer than the lower due to the way gravitational and centrifugal forces change with distance from Earth. His analysis included disturbances such as the gravitation of the Moon, wind and moving payloads up and down the cable. The weight of the material needed to build the elevator would have required thousands of Wikipedia:Space Shuttle trips, although part of the material could be transported up the elevator when a minimum strength strand reached the ground or be manufactured in space from asteroidal or lunar ore. In 1979, space elevators were introduced to a broader audience with the simultaneous publication of Wikipedia:Arthur C. Clarke's novel, Wikipedia:The Fountains of Paradise, in which engineers construct a space elevator on top of a mountain peak in the fictional island country of Taprobane (loosely based on Wikipedia:Sri Lanka, albeit moved south to the Equator), and Wikipedia:Charles Sheffield's first novel, Wikipedia:The Web Between the Worlds, also featuring the building of a space elevator. Three years later, in Wikipedia:Robert A. Heinlein's 1982 novel Friday the principal character makes use of the "Nairobi Beanstalk" in the course of her travels. In Wikipedia:Kim Stanley Robinson's 1993 novel Wikipedia:Red Mars, colonists build a space elevator on Mars that allows both for more colonists to arrive and also for natural resources mined there to be able to leave for Earth. In Wikipedia:David Gerrold's 2000 novel, Wikipedia:Jumping Off The Planet, a family excursion up the Ecuador "beanstalk" is actually a child-custody kidnapping. Gerrold's book also examines some of the industrial applications of a mature elevator technology. In a biological version, Wikipedia:Joan Slonczewski's novel The Highest Frontier depicts a college student ascending a space elevator constructed of self-healing cables of anthrax bacilli. The engineered bacteria can regrow the cables when severed by space debris. 21st century After the development of Wikipedia:carbon nanotubes in the 1990s, engineer Wikipedia:David Smitherman of Wikipedia:NASA/Marshall's Advanced Projects Office realized that the high strength of these materials might make the concept of an orbital skyhook feasible, and put together a workshop at the Wikipedia:Marshall Space Flight Center, inviting many scientists and engineers to discuss concepts and compile plans for an elevator to turn the concept into a reality.Science @ NASA, Audacious & Outrageous: Space Elevators, September 2000 The publication he edited, compiling information from the workshop, "Space Elevators: An Advanced Earth-Space Infrastructure for the New Millennium", provides an introduction to the state of the technology at the time (1999), and summarizes the findings. Another American scientist, Wikipedia:Bradley C. Edwards, suggested creating a long paper-thin ribbon using a carbon nanotube composite material. He chose the wide-thin ribbon-like cross-section shape rather than earlier circular cross-section concepts because that shape would stand a greater chance of surviving impacts by meteoroids. The ribbon cross-section shape also provided large surface area for climbers to climb with simple rollers. Supported by the Wikipedia:NASA Institute for Advanced Concepts, Edwards' work was expanded to cover the deployment scenario, climber design, power delivery system, orbital debris avoidance, anchor system, surviving Wikipedia:atomic oxygen, avoiding lightning and hurricanes by locating the anchor in the western equatorial Pacific, construction costs, construction schedule, and environmental hazards. To speed space elevator development, proponents have organized several competitions, similar to the Wikipedia:Ansari X Prize, for relevant technologies. Among them are Wikipedia:Elevator:2010, which organized annual competitions for climbers, ribbons and power-beaming systems from 2005 to 2009, the Robogames Space Elevator Ribbon Climbing competition, as well as NASA's Wikipedia:Centennial Challenges program, which, in March 2005, announced a partnership with the Wikipedia:Spaceward Foundation (the operator of Elevator:2010), raising the total value of prizes to US$400,000. The first European Space Elevator Challenge (EuSEC) to establish a climber structure took place in August 2011. In 2005, "the Wikipedia:LiftPort Group of space elevator companies announced that it will be building a carbon nanotube manufacturing plant in Wikipedia:Millville, New Jersey, to supply various glass, plastic and metal companies with these strong materials. Although LiftPort hopes to eventually use carbon nanotubes in the construction of a space elevator, this move will allow it to make money in the short term and conduct research and development into new production methods." Their announced goal was a space elevator launch in 2010. On February 13, 2006 the LiftPort Group announced that, earlier the same month, they had tested a mile of "space-elevator tether" made of carbon-fiber composite strings and fiberglass tape measuring wide and 1 mm (approx. 13 sheets of paper) thick, lifted with balloons. In 2007, Wikipedia:Elevator:2010 held the 2007 Space Elevator games, which featured US$500,000 awards for each of the two competitions, (US$1,000,000 total) as well as an additional US$4,000,000 to be awarded over the next five years for space elevator related technologies.Elevator:2010 – The Space Elevator Challenge. spaceward.org No teams won the competition, but a team from Wikipedia:MIT entered the first 2-gram (0.07 oz), 100-percent carbon nanotube entry into the competition.Spaceward Games 2007. The Spaceward Foundation Japan held an international conference in November 2008 to draw up a timetable for building the elevator. Lewis, Leo; News International Group; accessed September 22, 2008. In 2008 the book "Leaving the Planet by Space Elevator", by Dr. Brad Edwards and Philip Ragan, was published in Japanese and entered the Japanese best seller list. Edwards, Bradley C. and Westling, Eric A. and Ragan, Philip; Leasown Pty Ltd.; accessed September 26, 2008. This has led to a Japanese announcement of intent to build a Space Elevator at a projected price tag of a trillion yen (£5 billion/ $8 billion). In a report by Leo Lewis, Tokyo correspondent of The Times newspaper in England, plans by Shuichi Ono, chairman of the Japan Space Elevator Association, are unveiled. Lewis says: "Japan is increasingly confident that its sprawling academic and industrial base can solve those construction issues, and has even put the astonishingly low price tag of a trillion yen (£5 billion/ $8 billion) on building the elevator. Japan is renowned as a global leader in the precision engineering and high-quality material production without which the idea could never be possible." Google says that it has considered designing a Wikipedia:space elevator, but that the project is not currently feasible. In 2012, the Wikipedia:Obayashi Corporation announced that in 38 years it could build a space elevator using carbon nanotube technology. At 200 kilometers per hour, the design's 30-passenger climber would be able to reach the GEO level after a 7.5 day trip. No cost estimates, finance plans, or other specifics were made. This, along with timing and other factors, hinted that the announcement was made largely to provide publicity for the opening of one of the company's other projects in Tokyo. Physics of space elevators Apparent gravitational field A space elevator cable rotates along with the rotation of the Earth. Objects fastened to the cable will experience upward centrifugal force that opposes some of, all of, or more than, the downward gravitational force at that point. The higher up the cable, the stronger is the upward centrifugal force and the more it opposes the downward gravity. Eventually it becomes stronger than gravity above the geosynchronous level. Along the length of the cable, this (downward) actual gravity minus the (upward) centrifugal force is called the apparent gravitational field. The apparent gravitational field can be represented this way: :The downward force of actual gravity decreases with height: g = -G \cdot M/r^2 :The upward Wikipedia:centrifugal force due to the planet's rotation increases with height: a = \omega^2 \cdot r :Together, the apparent gravitational field is the sum of the two: : g = -G \cdot M/r^2 + \omega^2 \cdot r where :g'' is the acceleration of ''actual gravity or apparent gravity down (negative) or up (positive) along the vertical cable (m s−2), :a'' is the centrifugal acceleration up (positive) along the vertical cable (m s−2), :''G is the Wikipedia:gravitational constant (m3 s−2 kg−1) :M'' is the mass of the Earth (kg) :''r is the distance from that point to Earth's center (m), :ω'' is Earth's rotation speed (radian/s). At some point up the cable, the two terms (downward gravity and upward centrifugal force) equal each other; objects fixed to the cable there have no weight on the cable. This occurs at the level of the stationary orbit. This level (r1) depends on the mass of the planet and its rotation rate. Setting actual gravity and centrifugal acceleration equal to each other gives: : r_1 = (G \cdot M/\omega^2)^{1/3} On Earth, this level is above the surface, the level of geostationary orbit. Seen from a geosynchronous station, any object dropped off the tether from a point closer to Earth will initially accelerate downward. If dropped from any point above a geosynchronous station, the object would initially accelerate up toward space. Cable section Historically, the main technical problem has been considered the ability of the cable to hold up, with tension, the weight of itself below any particular point. The vertical point with the greatest tension on a space elevator cable is at the level of geostationary orbit, above the Earth's equator. This means that the cable material combined with its design must be strong enough to hold up the weight of its own mass from the surface up to 35,786 km. By making any cable larger in cross section at this level compared to at the surface, it can better hold up a longer length of itself. For a space elevator cable, an important design factor in addition to the material is how the cross section area tapers down from the maximum at 35,786 km to the minimum at the surface. To maximize the usable excess strength for a given amount of cable material, the cable's cross section area will need to be designed in such a way that at any given point, it is proportional to the force it has to withstand. Artuković, Ranko (2000). "The Space Elevator". zadar.net For such an idealized design without climbers attached, without thickening at high space-junk altitudes, etc., the cross-section will follow this differential equation: : \sigma \cdot dS = g \cdot \rho \cdot S \cdot dr or : dS/S = g \cdot \rho/\sigma \cdot dr or : dS/S = \rho/\sigma \cdot ( G \cdot M/r^2 - \omega^2 \cdot r ) \cdot dr where :''g is the acceleration along the radius (m·s−2), :S'' is the cross-section area of the cable at any given point r, (m2) and dS its variation (m2 as well), :ρ'' is the density of the material used for the cable (kg·m−3). :σ'' is the stress the cross-section area can bear without yielding (N·m−2=kg·m−1·s−2), its elastic limit. The value of ''g is given by the first equation, which yields: : \Delta\left[ \ln (S)\right]{}_{r_1}^{r_0} = \rho/\sigma \cdot \Delta\left[ G \cdot M/r + \omega^2 \cdot r^2/2 \right]{}_{r_1}^{r_0} , the variation being taken between r1 (geostationary) and r0 (ground). It turns out that between these two points, this quantity can be expressed simply as: \Delta\left[ \ln (S)\right] = \rho/\sigma \cdot g_0 \cdot r_0 \cdot ( 1 + x/2 - 3/2 \cdot x^{1/3} ) , or : S_0 = S_1.e^{\rho/\sigma \cdot g_0 \cdot r_0 \cdot ( 1 + x/2 - 3/2 \cdot x^{1/3} )} where x = \omega^2 \cdot r_0/g_0 is the ratio between the centrifugal force on the equator and the gravitational force. Cable material The free breaking length can be used to compare materials: it is the length of an un-tapered cylindrical cable at which it will break under its own weight under constant gravity. For a given material, that length is σ/ρ/g0. The free breaking length needed is given by the equation : \Delta\left[ \ln (S)\right] = \rho/\sigma \cdot g_0 \cdot r_0 \cdot ( 1 + x/2 - 3/2 \cdot x^{1/3} ) , where x = w^2 \cdot r_0/g_0. If one does not take into account the x'' factor (which reduces the strength needed by about 30 percent), this equation also says that the section ratio equals ''e (exponential one) when: : \sigma = \rho \cdot r_0 \cdot g_0. If the material can support a free breaking length of only one tenth this, the section needed at a geosynchronous orbit will be ''e''10 (a factor of 22026) times the ground section. Structure |thumb|right|One concept for the space elevator has it tethered to a mobile seagoing platform. There are a variety of space elevator designs. Almost every design includes a base station, a cable, climbers, and a counterweight. Earth's rotation creates upward Wikipedia:centrifugal force on the counterweight. The counterweight is held down by the cable while the cable is held up and taut by the counterweight. The base station anchors the whole system to the surface of the Earth. Climbers climb up and down the cable with cargo. Base station Modern concepts for the base station/anchor are typically mobile stations, large oceangoing vessels or other mobile platforms. Mobile base stations have the advantage over the earlier stationary concepts (with land-based anchors) by being able to maneuver to avoid high winds, storms, and Wikipedia:space debris. Oceanic anchor points are also typically in international waters, simplifying and reducing cost of negotiating territory use for the base station. Stationary land based platforms have simpler and less costly logistical access to the base. They also have an advantage of being able to be at high altitude, such as on top of mountains, or even potentially on high towers. This reduces influence from the atmosphere and how deep down into the Earth's gravity field the cable needs to extend, and so reduces the critical strength-to-density requirements for the cable material a bit (with all other design factors being equal). Cable thumb|right|[[Wikipedia:Carbon nanotubes are one of the candidates for a cable material]] A space elevator cable must carry its own weight as well as the additional weight of climbers. The required strength of the cable will vary along its length. This is because at various points it has to carry the weight of the cable below, or provide a downward force to retain the cable and counterweight above. Maximum tension on a space elevator cable is at geosynchronous altitude so the cable must be thickest there and taper carefully as it approaches Earth. Any potential cable design may be characterized by the taper factor – the ratio between the cable's radius at geosynchronous altitude and at the Earth's surface. The cable must be made of a material with a large tensile strength/density ratio. For example, the Edwards space elevator design assumes a cable material with a specific strength of at least 100,000 kN/(kg/m). This value takes into consideration the entire weight of the space elevator. An untapered space elevator cable would need a material capable of sustaining a length of of its own weight at Wikipedia:sea level to reach a Wikipedia:geostationary altitude of without yielding.This 4,960 km "escape length" (calculated by Wikipedia:Arthur C. Clarke in 1979) is much shorter than the actual distance spanned because centrifugal forces increase (and gravity decreases) dramatically with height: Therefore, a material with very high strength and lightness is needed. For comparison, metals like titanium, steel or aluminium alloys have breaking lengths of only 20–30 km. Modern fibre materials such as Wikipedia:kevlar, Wikipedia:fibreglass and carbon/graphite fibre have breaking lengths of 100–400 km. Nanoengineered materials such as Wikipedia:carbon nanotubes and, more recently discovered, Wikipedia:graphene ribbons (perfect two-dimensional sheets of carbon) are expected to have breaking lengths of 5000–6000 km at sea level, and also are able to conduct electrical power. For high specific strength, carbon has advantages because it is only the 6th element in the Wikipedia:periodic table. Carbon has comparatively few of the protons and neutrons which contribute most of the dead weight of any material. Most of the interatomic bonding forces of any element are contributed by only the outer few electrons. For carbon, the strength and stability of those bonds is high compared to the mass of the atom. The challenge in using carbon nanotubes remains to extend to macroscopic sizes the production of such material that are still perfect on the microscopic scale (as microscopic defects are most responsible for material weakness). The current (2014) carbon nanotube technology allows growing tubes up to a few tens of centimeters. |thumb|250px|right|A seagoing anchor station would incidentally act as a deep-water [[Wikipedia:seaport.]] Climbers thumb|right|250px|A conceptual drawing of a space elevator climber ascending through the clouds. A space elevator cannot be an elevator in the typical sense (with moving cables) due to the need for the cable to be significantly wider at the center than at the tips. While various designs employing moving cables have been proposed, most cable designs call for the "elevator" to climb up a stationary cable. Climbers cover a wide range of designs. On elevator designs whose cables are planar ribbons, most propose to use pairs of rollers to hold the cable with friction. Climbers must be paced at optimal timings so as to minimize cable stress and oscillations and to maximize throughput. Lighter climbers can be sent up more often, with several going up at the same time. This increases throughput somewhat, but lowers the mass of each individual payload.Lang, David D. Space Elevator Dynamic Response to In-Transit Climbers. thumb|right|250px|As the car climbs, the cable takes on a slight lean due to the Coriolis force. The top of the cable travels faster than the bottom. The climber is accelerated horizontally as it ascends by the Coriolis force which is imparted by angles of the cable. The lean-angle shown is exaggerated. The horizontal speed, i.e. due to orbital rotation, of each part of the cable increases with altitude, proportional to distance from the center of the Earth, reaching low Wikipedia:orbital speed at a point approximately 66 percent of the height between the surface and geostationary orbit (a height of about 23,400 km). A payload released at this point will go into a highly eccentric elliptical orbit, staying just barely clear from atmospheric reentry, with the Wikipedia:periapsis at the same altitude as LEO and the Wikipedia:apoapsis at the release height. With increasing release height the orbit becomes less eccentric as both periapsis and apoapsis increase, becoming circular at geostationary level. When the payload has reached GEO, the horizontal speed is exactly the speed of a circular orbit at that level, so that if released, it would remain adjacent to that point on the cable. The payload can also continue climbing further up the cable beyond GEO, allowing it to obtain higher speed at jettison. If released from 100,000 km, the payload would have enough speed to reach the asteroid belt. As a payload is lifted up a space elevator, it gains not only altitude, but horizontal speed (angular momentum) as well. This angular momentum is taken from the Earth's own rotation. As the climber ascends, it is initially moving slightly more slowly than each successive part of cable it is moving on to. This is the Wikipedia:coriolis force, the climber "drags" (Westward) on the cable as it climbs, slightly decreasing the Earth's rotation speed. The opposite process would occur for descending payloads, tilting the cable eastwards, and slightly increasing Earth's rotation speed. The overall effect of the centrifugal force acting on the cable causes it to constantly try to return to the energetically favorable vertical orientation, so after an object has been lifted on the cable the counterweight will swing back towards the vertical like an inverted pendulum. Space elevators and their loads will be designed so that the center of mass is always well-enough above the level of geostationary orbit"Why the Space Elevator's Center of Mass is not at GEO" by Blaise Gassend. Gassend.net. Retrieved on September 30, 2011. to hold up the whole system. Lift and descent operations must be carefully planned so as to keep the pendulum-like motion of the counterweight around the tether point under control. Climber speed is limited by the Coriolis force, available power, and by the need to ensure the climber's accelerating force does not break the cable. Climbers also need to maintain a minimum average speed in order to move material up and down economically and expeditiously. At the speed of a very fast car or train of it will take about 5 days to climb to geosynchronous orbit. Powering climbers Both power and energy are significant issues for climbers—the climbers need to gain a large amount of potential energy as quickly as possible to clear the cable for the next payload. Various methods have been proposed to get that energy to the climber: * Transfer the energy to the climber through Wikipedia:wireless energy transfer while it is climbing. * Transfer the energy to the climber through some material structure while it is climbing. * Store the energy in the climber before it starts – requires an extremely high Wikipedia:specific energy such as nuclear energy. * Solar power – power compared to the weight of panels limits the speed of climb. Wireless energy transfer such as laser power beaming is currently considered the most likely method. Using megawatt powered free electron or solid state lasers in combination with adaptive mirrors approximately wide and a photovoltaic array on the climber tuned to the laser frequency for efficiency. For climber designs powered by power beaming, this efficiency is an important design goal. Unused energy must be re-radiated away with heat-dissipation systems, which add to weight. Yoshio Aoki, a professor of precision machinery engineering at Wikipedia:Nihon University and director of the Japan Space Elevator Association, suggested including a second cable and using the conductivity of carbon nanotubes to provide power. Counterweight Several solutions have been proposed to act as a counterweight: *a heavy, captured Wikipedia:asteroid; *a Wikipedia:space dock, Wikipedia:space station or Wikipedia:spaceport positioned past geostationary orbit; or *a further upward extension of the cable itself so that the net upward pull is the same as an equivalent counterweight; *parked spent climbers that had been used to thicken the cable during construction, other junk, and material lifted up the cable for the purpose of increasing the counterweight.Edwards BC, Westling EA. (2002) ''The Space Elevator: A Revolutionary Earth-to-Space Transportation System. San Francisco, USA: Spageo Inc. ISBN 0-9726045-0-2. Extending the cable has the advantage of some simplicity of the task and the fact that a payload that went to the end of the counterweight-cable would acquire considerable velocity relative to the Earth, allowing it to be launched into interplanetary space. Its disadvantage is the need to produce greater amounts of cable material as opposed to using anything that has mass. Related concepts The conventional current concept of a "Space Elevator" has evolved from a static compressive structure reaching to the level of GEO, to the modern baseline idea of a static tensile structure anchored to the ground and extending to well above the level of GEO. In the current usage by practitioners (and in this article), a "Space Elevator" means the Tsiolkovsky-Artsutanov-Pearson type as considered by the International Space Elevator Consortium. This conventional type is a static structure fixed to the ground and extending into space high enough that cargo can climb the structure up from the ground to a level where simple release will put the cargo into an Wikipedia:orbit."CLIMB: The Journal of the International Space Elevator Consortium", Volume 1, Number 1, December 2011, This journal is cited as an example of what is generally considered to be under the term "Space Elevator" by the international community. http://www.isec.org/index.php?option=com_content&view=article&id=28&Itemid=31 Some concepts related to this modern baseline are not usually termed a "Space Elevator", but are similar in some way and are sometimes termed "Space Elevator" by their proponents. For example, Wikipedia:Hans Moravec published an article in 1977 called "A Non-Synchronous Orbital Skyhook" describing a concept using a rotating cable. The rotation speed would exactly match the orbital speed in such a way that the tip velocity at the lowest point was zero compared to the object to be "elevated". It would dynamically grapple and then "elevate" high flying objects to orbit or low orbiting objects to higher orbit. Other ideas use very tall compressive towers to reduce the demands on launch vehicles. The vehicle is "elevated" up the tower, which may extend as high as above the atmosphere, and is launched from the top. The original concept envisioned by Tsiolkovsky was a compression structure, a concept similar to an aerial mast. While such structures might reach space (100 km, 62 mi), they are unlikely to reach geostationary orbit. The concept of a Tsiolkovsky tower combined with a classic space elevator cable (reaching above the level of GEO) has been suggested. A tall towerBoucher, Marc. (September 1, 2009) Canadian Mini Space Elevator Paper Available – The Space Elevator Reference . Spaceelevator.com. Retrieved on September 30, 2011. to access near-space altitudes of has been proposed by Canadian researchers. The structure would be pneumatically supported and free standing with control systems guiding the structure's center of mass. Proposed uses include tourism and commerce, communications, wind generation and low-cost space launch. Other concepts related to a space elevator (or parts of a space elevator) include an Wikipedia:orbital ring, a pneumatic space tower, Scientists envision inflatable alternative to tethered space elevator, Wikipedia:ZDNet, June 17, 2009. Retrieved February 2013. a Wikipedia:space fountain, a Wikipedia:launch loop, a Skyhook, a Wikipedia:space tether, a space hoist and the Wikipedia:SpaceShaft.Space Shaft: Or, the story that would have been a bit finer, if only one had known... , "Knight Science Journalism Tracker (MIT)", July 1, 2009 Launching into deep space An object attached to a space elevator at a radius of approximately 53,100 km will be at Wikipedia:escape velocity when released. Transfer orbits to the L1 and L2 Wikipedia:Lagrangian points can be attained by release at 50,630 and 51,240 km, respectively, and transfer to lunar orbit from 50,960 km. At the end of Pearson's cable, the tangential velocity is 10.93 kilometers per second (6.79 mi/s). That is more than enough to escape Earth's gravitational field and send probes at least as far out as Wikipedia:Jupiter. Once at Jupiter, a Wikipedia:gravitational assist maneuver permits solar escape velocity to be reached. Extraterrestrial elevators A space elevator could also be constructed on other planets, asteroids and moons. A Martian tether could be much shorter than one on Earth. Mars' surface gravity is 38 percent of Earth's, while it rotates around its axis in about the same time as Earth. Because of this, Martian stationary orbit is much closer to the surface, and hence the elevator would be much shorter. Current materials are already sufficiently strong to construct such an elevator.Forward, Robert L. and Moravec, Hans P. (March 22, 1980) SPACE ELEVATORS. Carnegie Mellon University. "Interestingly enough, they are already more than strong enough for constructing skyhooks on the moon and Mars." Building a Martian elevator would be complicated by the Martian moon Phobos, which is in a low orbit and intersects the Equator regularly (twice every orbital period of 11 h 6 min). On the near side of the Moon, the strength-to-density required of the tether of a Wikipedia:lunar space elevator exists in currently available materials. A lunar space elevator would be about long. Since the Moon does not rotate fast enough, there is no effective lunar-stationary orbit, but the Wikipedia:Lagrangian points could be used. The near side would extend through the Earth-Moon L1 point from an anchor point near the center of the visible part of Earth's Moon. On the far side of the Moon, a lunar space elevator would need to be very long—more than twice the length of an Earth elevator—but due to the low gravity of the Moon, can also be made of existing engineering materials. Rapidly spinning asteroids or moons could use cables to eject materials to convenient points, such as Earth orbits; or conversely, to eject materials to send the bulk of the mass of the asteroid or moon to Earth orbit or a Wikipedia:Lagrangian point. Wikipedia:Freeman Dyson, a physicist and mathematician, has suggested using such smaller systems as power generators at points distant from the Sun where solar power is uneconomical. A space elevator using presently available engineering materials could be constructed between mutually tidally locked worlds, such as Pluto and Charon or the components of binary asteroid Antiope, with no terminus disconnect, according to Francis Graham of Kent State University. However, spooled variable lengths of cable must be used due to ellipticity of the orbits. Construction The construction of a space elevator would need reduction of some technical risk. Some advances in engineering, manufacturing and physical technology are required. Once a first space elevator is built, the second one and all others would have the use of the previous ones to assist in construction, making their costs considerably lower. Such follow-on space elevators would also benefit from the great reduction in technical risk achieved by the construction of the first space elevator. Construction is conceived as the deployment of a long cable from a large spool. The spool is initially parked in a geostationary orbit above the planned anchor point. When a long cable is dropped "down" (toward Earth), it must be balanced by balancing mass being dropped "up" (away from Earth) for the whole system to remain on the geosynchronous orbit. Earlier designs imagined the balancing mass to be another cable (with counterweight) extending upward, with the main spool remaining at the original geosynchronous orbit level. Most current designs elevate the spool itself as the main cable is paid out, a simpler process. When the lower end of the cable is so long as to reach the Earth (at the equator), it can be anchored. Once anchored, the center of mass is elevated more (by adding mass at the upper end or by paying out more cable). This adds more tension to the whole cable, which can then be used as an elevator cable. One plan for construction uses conventional rockets to place a "minimum size" initial seed cable of only 19,800 kg. This first very small ribbon would be adequate to support the first 619 kg climber. The first 207 climbers would carry up and attach more cable to the original, increasing its cross section area and widening the initial ribbon to about 160 mm wide at its widest point. The result would be a 750,000 kg cable with a lift capacity of 20 tons per climber. Safety issues and construction challenges For early systems, transit times from the surface to the level of geosynchronous orbit would be about five days. On these early systems, the time spent moving through the Wikipedia:Van Allen radiation belts would be enough that passengers would need to be protected from radiation by shielding, which adds mass to the climber and decreases payload. A space elevator would present a navigational hazard, both to aircraft and spacecraft. Aircraft could be diverted by Wikipedia:air-traffic control restrictions. All objects in stable orbits that have Wikipedia:perigee below the maximum altitude of the cable that are not synchronous with the cable will impact the cable eventually, unless avoiding action is taken. One potential solution proposed by Edwards is to use a movable anchor (a sea anchor) to allow the tether to "dodge" any space debris large enough to track. Impacts by space objects such as meteoroids, micrometeorites and orbiting man-made debris, pose another design constraint on the cable. A cable would need to be designed to maneuver out of the way of debris, or absorb impacts of small debris without breaking. Economics With a space elevator, materials might be sent into orbit at a fraction of the current cost. As of 2000, conventional rocket designs cost about US$25,000 per Wikipedia:kilogram (US$11,000 per pound) for transfer to geostationary orbit. Current proposals envision payload prices starting as low as $220 per kilogram ($100 per pound), similar to the $5–$300/kg estimates of the Wikipedia:Launch loop, but higher than the $310/ton to 500 km orbit quoted to Dr. Wikipedia:Jerry Pournelle for an Wikipedia:orbital airship system. Philip Ragan, co-author of the book "Leaving the Planet by Space Elevator", states that "The first country to deploy a space elevator will have a 95 percent cost advantage and could potentially control all space activities." See also * Wikipedia:Elevator:2010 – a space elevator prize competition * Wikipedia:Lunar space elevator for the Moon variant * Wikipedia:Space elevator construction discusses alternative construction methods of a space elevator. * Wikipedia:Space elevator economics discusses capital and maintenance costs of a space elevator. * Wikipedia:Space elevator safety discusses safety aspects of space elevator construction and operation. * Wikipedia:Space elevators in fiction * Wikipedia:Tether propulsion – for other transportation methods using long cables * Wikipedia:Non-rocket spacelaunch: ** Wikipedia:Launch loop – a hypervelocity belt system that forms a launch track at 80 km **Wikipedia:Lightcraft – an alternative method for moving materials or people ** Wikipedia:Space gun or Wikipedia:StarTram – among methods for launching materials ** Wikipedia:Space fountain – very tall structures using fast moving masses to hold it up ** Wikipedia:SpaceShaft – An atmospherically buoyant spar that could reach up to LEO and provide super-heavy lifting capacity. *Wikipedia:Space elevators in fiction References Further reading * Edwards BC, Ragan P. "Leaving The Planet By Space Elevator" Seattle, USA: Lulu; 2006. ISBN 978-1-4303-0006-9 * Edwards BC, Westling EA. The Space Elevator: A Revolutionary Earth-to-Space Transportation System. San Francisco, USA: Spageo Inc.; 2002. ISBN 0-9726045-0-2. *http://www.nss.org/resources/library/spaceelevator/2000-SpaceElevator-NASA-CP210429.pdf PDF. A conference publication based on findings from the Advanced Space Infrastructure Workshop on Geostationary Orbiting Tether "Space Elevator" Concepts, held in 1999 at the NASA Marshall Space Flight Center, Huntsville, Alabama. Compiled by D.V. Smitherman, Jr., published August 2000. *"The Political Economy of Very Large Space Projects" HTML PDF, John Hickman, Ph.D. Wikipedia:Journal of Evolution and Technology Vol. 4 – November 1999. *A Hoist to the Heavens By Bradley Carl Edwards *Ziemelis K. (2001) "Going up". In Wikipedia:New Scientist 2289: 24–27. Republished in SpaceRef. Title page: "The great space elevator: the dream machine that will turn us all into astronauts." *The Space Elevator Comes Closer to Reality . An overview by Leonard David of space.com, published March 27, 2002. * Krishnaswamy, Sridhar. Stress Analysis — The Orbital Tower (PDF) * Wikipedia:LiftPort's Roadmap for Elevator To Space SE Roadmap (PDF) * Space Elevators Face Wobble Problem : New Scientist * Alexander Bolonkin, “Non Rocket Space Launch and Flight”. Elsevier, 2005. 488 pgs. ISBN 978-0-08044-731-5 .http://www.archive.org/details/Non-rocketSpaceLaunchAndFlight, External links * The Space Elevator Reference * Space Elevator Engineering-Development wiki * Audacious & Outrageous: Space Elevators * Ing-Math.Net (Germany) – Ing-Math.Net (German Max-Born Space Elevator Team 2006) (German) * Project of the Scientific Workgroup for Rocketry and Spaceflight(WARR) (German) * The Economist: Waiting For The Space Elevator (June 8, 2006 – subscription required) * CBC Radio Quirks and Quarks November 3, 2001 Riding the Space Elevator * Times of London Online: Going up ... and the next floor is outer space * [http://www.islandone.org/LEOBiblio/CLARK1.HTM The Space Elevator: 'Thought Experiment', or Key to the Universe?]. By Sir Arthur C. Clarke. Address to the XXXth International Astronautical Congress, Munich, September 20, 1979. * The Space Elevator – Physical Principles The math and the numbers for actual materials. Wikipedia:Template:Space elevator Wikipedia:Template:Non-rocket spacelaunch Wikipedia:Template:Emerging technologies Category:Space elevator Category:Exploratory engineering Category:Megastructures Category:Space colonization Category:Spacecraft propulsion Category:Spaceflight technologies Category:Vertical transport devices Category:Space access Category:Hypothetical technology Category:Emerging technologies Category:Futurology Category:Globalization Category:Hypotheses Category:Imagineering Category:Transport technologies Category:Utopian concepts Category:Technology Category:Technology forecasting Category:Upcoming products Category:Technology by type Category:Technological change Category:Preparation for the future Category:Future Category:Prediction Category:Social change Category:Fictional locations Category:Social concepts Category:Mythological places